Join/Renew (2024 Dues)
News from https://groups.io/g/SanDiegoRegionBirding

Tropical Storm Hillary?? ; more on the maybe American Oystercatcher (long)

2:34 pm

As many of you know, the weather service is upping the chances that the moisture-rich flow associated with now-forming tropical system Hillary off west Mexico may get up to southern California around Sunday-Tuesday, bringing the potential for a fair bit of rain. The actual storm system will have long since fallen apart well to our south and what winds we do get are forecast to be OFFshore easterlies. But, if enough tropical flow indeed makes it up here, then folks should probably be on the lookout for frigatebirds. (Of course, now that I have posted this report already on Weds, that will guarantee that the whole system and forecast fizzles between now and then….)
For those interested in some interesting exchanges between several folks about the potential juvenile American Oystercatcher at La Jolla, here they are in chronological order. Perhaps more than you want to know. Four people have scored the bird so far: one 29-31, one 30, one 31, and one 30-32. Comments follow here:
I get 30-32, but the uppertail coverts are difficult to score (Jehl did
not include juveniles). This fits in with scores Jehl found from the
Coronados south (see his figure 3 for ranges of scores by location).
Really hard to know what influences each character. It’s probably as
frazeri as they get in So. Cal., except the birds at the Salton Sea. Neat bird
no doubt!


The upper and under tail coverts were the squishy ones. For Under I put
mainly white not equal black and white. That bumps it up. But limited
white in secondaries and rectrices seem far below (lower #) where other
characters fall on this bird. F1s are not always intermediate, and Jehl
notes that. But I did not find if mixed pairs produced young that are
mixed black birds and white birds. Seems odd these two La Jolla juvs are hanging
together.



There’s that 1920s or whatever year photo in the Jehl article, wherever it was in w. Mexico, showing two young fuzzy chicks, one totally dark and one with
entire white lower breast and belly, which presumably are from the same
clutch!
My scoring for the new oystercatcher is 30. By my
scoring, it falls just inside the “good enough” American category, but I
will also say that some things may change when the bird molts into
adult-like plumage, though most likely further into American as opposed
to more hybrid-like.
I
will also say, having not read the full paper in a long time, that it
appears that Jehl was thinking that the transition from the Black
phenotype into the American phenotype was continuous and based on
quantitative genetics.  If one looks instead at the genetics of the
Variable Oystercatcher from New Zealand (and my guess is that the
American/Black genetics are similar), it appears that one locus with
full dominance controls most of the change, with additional loci
controlling only subtle variation in the white-bellied phenotype.  The
result is that even in the first generation of hybrids, ALL hybrid young
will be white-bellied and looking essentially like American
Oystercatchers. If looking at things this way, the
white-bellied/black-bellied distinction may not closely reflect the
underlying genetics, especially for the white-bellied birds. This is
also likely the explanation for why the vast majority of hybrid-like
birds score in the 25-30 range rather than showing a smooth transition
from Black to American.  The real problem with this is that birds that
look like Americans may not be dominated by American genetics.

Upper tail coverts - 2 - nearly equally black and white

Tail - 2 - basal quarter of rectrices white

Chest - 3 - black chest band bordered by ragged edge on upper breast

Belly - 6 - entirely white, as in palliatus

Under tail coverts - 2 - nearly equally black and white
Thighs – 4 – entirely white, as in palliatus

Greater secondary coverts – 3 – 6-15 mm

Extent of white wing stripe - 1 - white markings confined to inner half of secondaries

Underwing coverts - 3 - mainly white
Axillars – 4 – white, as in palliatus

TOTAL = 30 (so, right on the edge, but just inside American based on Jehl’s index)
If my observation of the chicks this year at Cabrillo NM from the
“BLOY” x hybrid AMOY x BLOY showed anything it was that even a bird
scoring a a hybrid crossing with what appeared to be a pure BLOY can
produce AMOY appearing offspring. Too bad they didn’t last more than a
couple of days as it would have been interesting to see where they ended
up scoring. 
Perhaps it is time
for some genetics work on the northern edge of the BLOY range and the
southern edge of the AMOY’s Pacific range. This La Jolla bird was
leaving plenty of samples all over the rocks yesterday. What would be
even more interesting would be to collect some from the “BLOY” juvenile
it is associating with. Could be siblings for all we know.
If the control of the white versus black on the belly is
full dominance at one locus, a pure American crossing with a pure Black
would produce ONLY American-like young (in Variable Oystercatcher, the
white-bellied form is dominant, and this appears to be true for American
and Black as well), and a pure Black mating with a F1 hybrid between
American and Black (which will always be white-bellied) would produce
on-average 50% American-like young and 50% Black-like young.
It
would be ideal if somebody could get DNA of this La Jolla bird, combined with the extensive
set of photos that show all of the characters.
I
should also clarify that three of the characters can be difficult to
score based on photos.  First, it is impossible to measure from photos
the width of bar across the greater secondary-coverts, so I generally
rate this at 6-15 mm, versus >15 mm, but it is hard to really know. 
For perspective, 6 mm is about a quarter of an inch and 15 mm is between
2/3 and 3/4 of an inch, which is pretty broad for a bird of this size. 
Second and third, it is hard to really know how much white is on the
remiges or rectrices because they are covered by the coverts (secondary
or uppertail).  Even in a good photo of the spread wing it is hard to
know if there is white at the bases of the secondaries, or even the
primaries, because they are covered and impossible to see.  I find that I
score very few birds with any white at all on the outer secondaries or
primaries, but it is possible that it is there, yet obscurred.

–Paul Lehman, San Diego